OLAP Internals
Why are OLAP servers so much better at processing ad hoc queries?
The same sorts of aggregations that might take hours to process in an ordinary relational database can be processed in milliseconds in an OLAP cube.
What are the underlying mechanics of the model that make that possible?

OLAP Using ASP.NET (it is 2004, but it describes cube schema from our usage examples, skip 2 last sections)

Codd's Paper (OLAP description by Dr. E.F. Codd, a father of relational model and OLAP term)